A DC power source unit supplies a DC voltage to a power tool through an adapter when a power switch of the tool is turned ON and also charges a battery pack used as an alternative power source of the tool when the power tool is not operated. A minimum battery temperature gradient amongst battery temperature gradients ever computed is used to determine whether the battery pack has cooling effect when forcibly cooled by a cooling fan or whether the battery pack has reached a full charge condition. When charging the battery pack is interrupted due to operation of the power tool, computing the battery temperature gradient is halted for some time or determination of the battery pack's full charge condition is not performed for some time in order to prevent the minimum battery temperature from being updated during interruption of charging of the battery pack.